This invention relates to monopulse radars, and in particular to means for effecting simultaneous nulling in the sum and difference patterns of a monopulse phased array antenna using one set of adaptive weights shared by both channels.
In the past few years, considerable research and development has been expended on adaptive antennas. Communications and sonar systems have reaped some of the benefits of adaptive technology while radars generally lag behind. Some of the reasons for this dichotomy are many adaptive techniques are not suited for microwave frequencies; radars have large antennas, hence more adaptive loops; and a radar has tight time constraints for detecting and tracking targets. Consequently, only a handful of radars incorporating sidelobe cancelling techniques exist today. A fully adaptive phased array antenna is not practical to build at this time.
Monopulse radars add additional problems to the adaptive processing. The problems stem from the coextistence of a sum and difference pattern. A monopulse phased array antenna uses two antenna patterns: one to detect and range a target and a second to determine a target's angular location. Most adaptive antenna research has ignored the difference pattern, even though both patterns must have a null in the direction of interference to enhance the radar's performance. Adjusting the far field sum pattern to place a null in the direction of interference will not place a null in the difference pattern too. Hence, either the sum and difference channels are adapted separately or a technique of simultaneously nulling in both channels is used. Current research has assumed the first approach is the only feasible technique, and the possibility of simultaneous nulling has been ignored.
The current state of the art relating to radar system null pattern generation techniques is illustrated by U.S. Pat. No. 4,214,244 entitled Null Pattern Technique For Reduction of An Undesirable Interference Signal issued to Michael W. McKay July 22, 1980, U.S. Pat. No. 4,224,870 entitled Null Steering Antenna issued to Gene R. Marrs Sept. 30, 1980, and U.S. Pat. No. 4,298,873 entitled Adaptive Steerable Null Antenna Processor issued to Eugene L. Roberts Nov. 3, 1981. Adaptive nulling in these and other prior art systems is done with two separate sets of adaptive weights: one in the sum channel and one in the difference channel. The technique of the present invention allows nulling with one set of adaptive weights shared by the two channels thereby greatly reducing hardware complexity, computer software and the cost for adaptive processing.